A typical rotary drag type drill bit comprises a bit body which may be formed with a series of upstanding, generally radially extending blades. Each blade is typically provided with a series of cutting elements positioned such that, in use, when a weight is applied to the drill bit whilst the bit is driven for rotation about its axis, the cutting elements bear against the adjacent formation, scraping, gouging, abrading, cutting or otherwise removing the formation material, and thereby extending the length of a borehole. Often, a fluid is pumped into the borehole, for example being supplied through nozzles formed in the drill bit, and serving to clean and cool the cutting elements and to carry away the formation material removed in this fashion.
One common form of cutting element comprises a table or layer of a superhard material such as polycrystalline diamond bonded to a substrate of a less hard material such as tungsten carbide. The cutters are typically sintered under high temperature, high pressure conditions. After sintering, further procedures may be undertaken to remove a binder or catalysing material from parts thereof, and to clean and shape the cutting element.
In use, as a result of their engagement with the formation material, the cutting elements affixed to a drill bit will become worn, reducing the effectiveness of the drill bit. A point will be reached beyond which the drill bit requires replacement. Since replacement of a drill bit requires the drilling operation to be stopped and the drill string to which the drill bit is connected to be withdrawn from the borehole, before the drill bit can be replaced and introduced into the borehole, it will be appreciated that the act of replacement of a drill bit causes significant delays and incurs significant cost. It is desirable, therefore, to extend the working life of a drill bit which can be achieved by extending the working life of the cutting elements used on a drill bit. Consequently, replacement of a drill bit may be undertaken less frequently.
U.S. Pat. No. 5,025,874, U.S. Pat. No. 5,217,081, U.S. Pat. No. 6,065,554 and U.S. Pat. No. 6,986,297 all describe cutting elements for use on drill bits for the formation of boreholes. In the U.S. Pat. No. 5,025,874 arrangement, a layer of a superhard material is formed within a substrate such that the layer is, in effect, positioned between and bonded to two substrates. The element can then be divided to form two separate cutting elements. U.S. Pat. No. 5,217,081 describes a cutting element in which a substrate thereof includes cobalt rich and cobalt lean carbide regions. U.S. Pat. No. 6,065,554 describes a cutting element comprising a primary cutter including a table of superhard material provided on a substrate. A recess is formed in the front, superhard material covered face of the primary cutter in which an insert is provided, the insert itself having a superhard material front face displaced forwardly of the front face of the primary cutter. A similar structure to that of U.S. Pat. No. 6,065,554 is described in U.S. Pat. No. 6,986,297.
U.S. Pat. No. 6,258,139, US2013/0151848, GB2304358 and U.S. Pat. No. 5,979,578 all describe cutting element arrangements in which separate, distinct hard material regions are provided. The regions are typically provided by sintering simultaneously with one another.